Semiconductor device

ABSTRACT

A semiconductor device includes: a substrate; a buffer layer including GaN formed on the substrate, wherein: surfaces of the buffer layer are c facets of Ga atoms; a channel layer including GaN or InGaN formed on the buffer layer, wherein: surfaces of the channel layer are c facets of Ga or In atoms; an electron donor layer including AlGaN formed on the channel layer, wherein: surfaces of the electron donor layer are c facets of Al or Ga atoms; a source electrode and a drain electrode formed on the electron donor layer; a cap layer including GaN or InGaAlN formed between the source electrode and the drain electrode, wherein: surfaces of the cap layer are c facets of Ga or In atoms and at least a portion of the cap layer is in contact with the electron donor layer; and a gate electrode formed at least a portion of which is in contact with the cap layer.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. Ser. No.09/733,593, entitled “GAN-Based HFET Having a Surface-Leakage ReducingCap Layer”, filed Dec. 8, 2000, and which claims priority to JapaneseApplication No. 11-349330, filed Dec. 8, 1999.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a semiconductor device, and moreparticularly to a field-effect transistor having a heterostructure of aGallium nitride-based semiconductor which is generally represented asIn_(x)Al_(y)Ga_(1−x-y)N (where 0#X#1, 0#Y#1).

[0004] 2. Description of the Related Art

[0005] A Gallium nitride-based semiconductor such as GaN, AlGaN, InGan,InAlGaN or the like has high dielectric breakdown field, high thermalconductivity and a high electron saturation velocity, and thus ispromising as a material for a high-frequency power device. Particularlyin a semiconductor device having an AlGaN/GaN heterojunction structure,electrons accumulate at a high density in a heterojunction interfacebetween AlGaN and GaN, and a so-called two-dimensional electron gas isformed. This two-dimensional electron gas exists in a spatiallyseparated state from donor impurities added to AlGaN, and thus showshigh electron mobility. A field-effect transistor having such aheterostructure is produced so that source resistance can be reduced.Moreover, a distance d from a gate electrode to the two-dimensionalelectron gas is typically as short as tens of nm, and thus, even if agate length Lg is as short as about 100 nm, the ratio of the gate lengthLg to the distance d (i.e., aspect ratio) Lg/d, can be increased from 5to about 10. Accordingly, semiconductor devices having a heterostructurehave a superior feature in that a field-effect transistor which has aninsignificant short-channel effect and satisfactory saturation propertycan be readily produced. Moreover, a two-dimensional electron of theAlGaN/GaN-based heterostructure has an electron velocity in a high fieldregion of about 1×10⁵V/cm, which is twice or more than the speed ofAlGaAs/InGaAs-heterostructure currently prevalent as a high-frequencytransistor, and thus, is expected to be applied to high-frequency powerdevices.

[0006] A conventional semiconductor device 900 is shown in FIG. 9. Thesemiconductor device 900 is formed on a sapphire substrate or SiCsubstrate 901, on which the following layers are sequentially laminated:a buffer layer 902 including GaN; a channel layer 903 formed of GaN orInGaN; and an electron donor layer 904 including AlGaN. A sourceelectrode 906, a gate electrode 907 and a drain electrode 908 areprovided on the electron donor layer 904.

[0007] This AlGaN/GaN-based heterostructure is typically formed on asapphire substrate or SiC substrate 901 composed of a (0001) facet (cfacet), through a crystal growth process using a metal-organic chemicalvapor deposition method or a molecular beam epitaxy method. In the caseof forming the buffer layer 902 including GaN on the sapphire substrateor SiC substrate 901, it is necessary to thickly form the buffer layer902 in order to account for a great difference in lattice constantbetween the substrate 901 and the buffer layer 902. This is because thestrain due to a lattice mismatch between the buffer layer 902 and thesubstrate 901 is sufficiently reduced by forming the buffer layer 902 soas to have a relatively large thickness. By forming the electron donorlayer 904 containing AlGaN to which n-type impurities such as Si or thelike are added so as to have a thickness on the order of tens of nm onthis thick buffer layer 902, a two-dimensional electron gas (i.e.,channel layer 903) is formed in the buffer layer 902 which has a greatelectron affinity in the heterointerface between AlGaN and GaN due tothe effects of selective doping. The crystal facet of a heterostructureformed by an MOCVD (metal-organic chemical vapor deposition) method, istypically composed of a facet of Ga, which is an III group element. Thistwo-dimensional electron gas is susceptible to the effects ofpiezo-polarization in a c axis direction due to tensile stress imposedon AlGaN, in addition to a difference in spontaneous polarizationbetween AlGaN (included in the electron donor layer 904) and GaN(included in the buffer layer 902). Thus, electrons accumulate at adensity which is higher than a value which would be expected from thedensity of the n-type impurities added to the electron donor layer 904.When Al composition of AlGaN of the electron donor layer 904 is 0.2 to0.3, electron density of the channel layer 903 is about 1×10¹³/cm²,which is about 3 times the density of a GaAs-based device. Since thetwo-dimensional electron gas of such a high density is accumulated, thesemiconductor device 900 used as a GaN-based heterostructurefield-effect transistor (FET) is considered as a highly promising powerdevice.

[0008] However, the conventional semiconductor device 900 has a numberof problems as follows: (1) due to the imperfectness of crystal growthtechniques and their associated processes, a satisfactory crystal cannot be obtained; and (2) in the case of involving an etching process,the device properties may be deteriorated due to damage inflicted by theetching process, and thus, the expected power characteristics may not besufficiently realized.

[0009] One of the problems related to the crystal growth is associatedwith the fact that the undoped GaN included in the buffer layer 902typically represents an n-type and the carrier density may be as high asabout 10¹⁶/cm³ or more. This is presumably because the constituentnitrogen (N) atoms are released during the crystal growth, and thus,vacancies are liable to be formed. When there are such residualcarriers, the leakage current component via the GaN buffer layer 902 ofthe device becomes greater. In particular, when operating the device ata high temperature, deteriorations in the element properties such asaggravation of pinch-off characteristics may occur. As for an isolationproblem, when forming a plurality of GaN-based heterostructure FETs onthe same substrate, the FETs interfere with each other to hinder normaloperation. When the gate electrode 907 is further provided above thisGaN buffer layer 902, a problem such as an increase of a gate leakagecurrent, a drop in the voltage breakdown level of the device or the likemay arise.

[0010] As for problems associated with etching process technique, afacet of GaN (included in the buffer layer 902) or AlGaN (included inthe electron donor layer 904) may be damaged. Since GaN and AlGaN aredifficult to remove or trim by means of wet etching, dry etching istypically performed for the etching process. However, a leakage currentis likely to flow in the surface of the buffer layer 902 or the electrondonor layer 904 due to the damage inflicted on the surface of the bufferlayer 902 or the electron donor layer 904. It is considered that inparticular, shortage of nitrogen on the surface increases theconductivity of the surface of the buffer layer 902 exposed by theetching, thereby increasing the leakage current.

SUMMARY OF INVENTION

[0011] In one aspect of the invention, a semiconductor device includes:a substrate; a buffer layer including GaN formed on the substrate,wherein: surfaces of the buffer layer are c facets of Ga atoms; achannel layer including GaN or InGaN formed on the buffer layer,wherein: surfaces of the channel layer are c facets of Ga or In atoms;an electron donor layer including AlGaN formed on the channel layer,wherein: surfaces of the electron donor layer are c facets of Al or Gaatoms; a source electrode and a drain electrode formed on the electrondonor layer; a cap layer including GaN or InGaAlN formed between thesource electrode and the drain electrode, wherein: surfaces of the caplayer are c facets of Ga or In atoms and at least a portion of the caplayer is in contact with the electron donor layer; and a gate electrodeformed at least a portion of which is in contact with the cap layer.

[0012] In one embodiment of the invention, at least a portion of thegate electrode may be formed so as to contact the electron donor layer.

[0013] In another embodiment of the invention, the gate electrode may beformed on the cap layer.

[0014] In still another embodiment of the invention, the cap layer mayinclude InGaAlN, the cap layer may have a composition which issubstantially lattice matched with the buffer layer in a c facet and theelectron donor layer may be formed so that an absolute value of amagnitude of a polarization occurring within the cap layer is smallerthan that of a magnitude of a polarization occurring within the electrondonor layer.

[0015] In still another embodiment of the invention, an n-type impuritymay be added to part or whole of the cap layer.

[0016] In still another embodiment of the invention, the gate electrodemay be positioned closer to the source electrode than to the drainelectrode.

[0017] In still another embodiment of the invention, the gate electrodemay have a surface area which is larger than that of the cap layer.

[0018] In still another embodiment of the invention, the gate electrodemay be positioned in a region where the cap layer is reduced inthickness or removed.

[0019] In still another embodiment of the invention, the gate electrodemay be formed on a side of the cap layer closer to the source electrode,and the cap layer may be formed between the gate electrode and the drainelectrode.

[0020] In still another embodiment of the invention, the cap layer mayinclude a semiconductor layer formed on the electron donor layer and aninsulating film formed on the semiconductor layer.

[0021] The present invention having the above-described structureenhances a barrier height of Schottky junction, thereby providing asemiconductor device, which is capable of: reducing the leakage currentas well as preventing an increase of the source resistance; and/orimproving the voltage breakdown level as well as preventing an increaseof the source resistance. Moreover, a region occupied by a cap layerbetween a gate electrode and a drain electrode is made larger. Such astructure allows the voltage breakdown level of a semiconductor deviceto be improved.

[0022] In one aspect of the invention, a semiconductor device includes:a substrate; a buffer layer including AlGaN formed on the substrate,wherein: surfaces of the buffer layer are c facets of N atoms; anelectron donor layer including AlGaN formed on the buffer layer,wherein: surfaces of the electron donor layer are c facets of N atoms; achannel layer including GaN or InGaN formed on the electron donor layer,wherein: surfaces of the channel layer are c facets of N atoms; a sourceelectrode and a drain electrode formed on the channel layer; a cap layerincluding AlGaN formed between the source electrode and the drainelectrode, wherein: surfaces of the cap layer are c facets of N atomsand at least a portion of the cap layer is in contact with the channellayer; and a gate electrode formed at least a portion of which is incontact with the cap layer.

[0023] In one embodiment of the invention, the gate electrode may beformed so that at least a portion of which is in contact with thechannel layer.

[0024] In another embodiment of the invention, the gate electrode may beformed on the cap layer.

[0025] In still another embodiment of the invention, the gate electrodemay be positioned closer to the source electrode than to the drainelectrode.

[0026] In still another embodiment of the invention, the gate electrodemay have a surface area which is larger than that of the cap layer.

[0027] In still another embodiment of the invention, the gate electrodemay be positioned in a region where the cap layer is reduced inthickness or removed.

[0028] In still another embodiment of the invention, the gate electrodemay be formed on a side of the cap layer closer to the source electrode,and the cap layer may be formed between the gate electrode and the drainelectrode.

[0029] In still another embodiment of the invention, the cap layer mayinclude a semiconductor layer formed on the electron donor layer and aninsulating film formed on the semiconductor layer.

[0030] The present invention having the above-described structureenhances a barrier height of Schottky junction, thereby providing asemiconductor device, which is capable of: reducing the leakage currentas well as preventing an increase of the source resistance; and/orimproving the voltage breakdown level as well as preventing an increaseof the source resistance. Moreover, a region occupied by a cap layerbetween a gate electrode and a drain electrode is made larger. Such astructure allows the voltage breakdown level of a semiconductor deviceto be improved.

[0031] Thus, the invention described herein makes possible theadvantages of: (1) providing a semiconductor device (GaN-basedheterostructure FET) in which a surface leakage current caused byresidual carriers resulting from defects or damage accidentally causedin the interior or surface of the GaN layer is significantly reduced;and (2) providing a semiconductor device (GaN-based heterostructure FET)having a reduced surface leakage current and improved voltage breakdownlevel.

[0032] These and other advantages of the present invention will becomeapparent to those skilled in the art upon reading and understanding thefollowing detailed description with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF DRAWINGS

[0033]FIG. 1A is a cross-sectional view explaining a field-effecttransistor according to Example 1 of the present invention.

[0034]FIG. 1B is a plan view illustrating a field-effect transistoraccording to Example 1 of the present invention.

[0035]FIG. 2 is a potential graph relative to Example 1 of the presentinvention.

[0036]FIG. 3 is a graph showing dependency of sheet electron density andpeak potential on a thickness of GaN cap layer relative to Example 1 ofthe present invention.

[0037]FIG. 4 is a cross-sectional view illustrating a field-effecttransistor according to a variant of Example 1 of the present invention.

[0038]FIG. 5 is a cross-sectional view illustrating a field-effecttransistor according to another variant of Example 1 of the presentinvention.

[0039]FIG. 6A is a cross-sectional view illustrating a field-effecttransistor according to Example 2 of the present invention.

[0040]FIG. 6B is a cross-sectional view illustrating a field-effecttransistor according to Example 2 of the present invention.

[0041]FIG. 6C is a cross-sectional view illustrating a field-effecttransistor according to Example 2 of the present invention.

[0042]FIG. 6D is a cross-sectional view illustrating a field-effecttransistor according to Example 2 of the present invention.

[0043]FIG. 6E is a cross-sectional view illustrating a field-effecttransistor according to Example 2 of the present invention.

[0044]FIG. 7 is a cross-sectional view illustrating a field-effecttransistor according to Example 3 of the present invention.

[0045]FIG. 8A is a cross-sectional view illustrating a field-effecttransistor according to a variant of Example 3 of the present invention.

[0046]FIG. 8B is a cross-sectional view illustrating a field-effecttransistor according to a variant of Example 3 of the present invention.

[0047]FIG. 8C is a cross-sectional view illustrating a field-effecttransistor according to a variant of Example 3 of the present invention.

[0048]FIG. 8D is a cross-sectional view illustrating a field-effecttransistor according to a variant of Example 3 of the present invention.

[0049]FIG. 8E is a cross-sectional view illustrating a field-effecttransistor according to a variant of Example 3 of the present invention.

[0050]FIG. 9 is a cross-sectional view illustrating a conventionalfield-effect transistor.

DETAILED DESCRIPTION EXAMPLE 1

[0051] A semiconductor device according to Example 1 of the presentinvention will be described with reference to the figures. FIG. 1A is across-sectional view of a field-effect transistor (FET) 100 according toExample 1 of the present invention and FIG. 1B is a plan view thereof.The field-effect transistor 100 is formed on a substrate 101 composed ofsapphire or SiC, on which the following layers are sequentiallylaminated: a GaN buffer layer 102 having a thickness of about 2-3 μm; achannel layer 103 formed of GaN or InGaN; an n-type AlGaN electron donorlayer 104 having an AlN component ratio of about 0.15 to 0.5, to whichan n-type impurity such as Si is added at a density of about 2×10¹⁸cm⁻³;and a GaN cap layer 105 having a thickness of about 10-20 nm. The GaNcap layer 105 is selectively etched so as to leave only a centralportion thereof. A gate electrode 107 is formed on the GaN cap layer105. A source electrode 106 and a drain electrode 108 are formed,adjacent to the gate electrode 107, on the surface of the AlGaN electrondonor layer 104 exposed after portions of the GaN cap layer 105 areremoved. The surfaces of each nitride layer are composed of c facets ofa III group element.

[0052] As shown in FIG. 1B, in the periphery of an element structuralregion 109, a separation region 110 surrounding the element structuralregion 109 is formed by means of a method which does not involveetching, e.g., ion implantation. The GaN cap layer 105 is formed in anarea which is larger than the gate electrode 107. Moreover, the GaN caplayer 105 is formed so as not to contact the source electrode 106 andthe drain electrode 108. The GaN cap layer 105 functions to enhance aneffective barrier height (peak potential) of a Schottky electrode, asaccounted for by a difference between the magnitude of polarizationoccurring in the GaN cap layer 105 and that occurring in the AlGaNelectron donor layer 104.

[0053] Next, the influence of polarization occurring when stress isimposed on the field-effect transistor 100 having the above-describedstructure will be described.

[0054] Since the GaN buffer layer 102 is sufficiently thick to reduce acompression strain due to lattice mismatching, no piezo-polarizationoccurs due to the strain, but only spontaneous polarization occurs. Onthe other hand, the AlGaN electron donor layer 104 is subjected totensile strain, and substantial piezo-polarization occurs therein inaddition to the spontaneous polarization. This polarization occurs in ac axis direction of the substrate 101, i.e., a direction perpendicularto the upper surface of the substrate 101. FIG. 2 shows calculationresults of theoretical potential along a depth direction, using aninterface between the GaN cap layer 105 and the gate electrode 107 ofthe semiconductor device 100 shown in FIG. 1A as a point of reference(zero distance). The calculation takes into consideration theaforementioned influence from polarization.

[0055] In FIG. 2, the thickness of the GaN cap layer 105 is set to 10 nmand a gate voltage is set to OV. A potential difference occurs in theGaN cap layer 105 due to the influence of polarization. As a result, thepotential in a heterointerface with the AlGaN electron donor layer 104(the peak potential shown in FIG. 2) is increased, thereby increasingthe effective height of the Schottky barrier.

[0056]FIG. 3 shows calculation results of theoretical changes (marked byx in FIG. 3) in the effective barrier height (peak potential) where thethickness of the GaN cap layer 105 is varied from 0 to 20 nm, andchanges (denoted by “O” symbols in FIG. 3) in the electron densityaccumulating at the heterointerface between the GaN cap layer 105 andthe AlGaN electron donor layer 104.

[0057] As shown in FIG. 3, while the effectual barrier height (peakpotential) of the Schottky electrode gradually increases as thethickness of the GaN cap layer 105 increases, the electron densityaccumulating in the heterointerface between the GaN cap layer 105 andthe AlGaN electron donor layer 104 decreases. The reason why the peakpotential increases is that the barrier height of the Schottky electrodeto the GaN cap layer 105 remains constant, whereas a potentialdifference occurring in the GaN cap layer 105 increases along with anincrease of a film thickness of the GaN cap layer 105. Thus, theaddition of the GaN cap layer 105 effectively increases the peakpotential. The electron density decreases as the thickness of the GaNcap layer 105 increases because a reverse bias is applied to the gateelectrode by the potential difference residing in the GaN cap layer 105.

[0058] As described above, the provision of the GaN cap layer 105increases the peak potential and reduces the electron densityaccumulating at the heterointerface. All of these factors contribute tothe high voltage breakdown level of the resultant field-effecttransistor. However, the leakage current includes a component whichflows along the surfaces of the buffer layer 102. In particular, in thecase of a material which creates a donor responsive to depletion ofnitrogen atoms on the surfaces, e.g., GaN included in the buffer layer102, it is important to reduce the aforementioned component in theleakage current. Moreover, a reduction in the electron densityaccumulating at the heterointerface leads to: an increase of resistancein a region where there is the GaN cap layer 105; an increase of sourceresistance of the field-effect transistor; and a deterioration inperformance of the transistor.

[0059] In the field-effect transistor 100 according to the presentinvention, the GaN cap layer 105 in the region between a gate and asource is removed (i.e., the source electrode 106 and the cap layer 105do not directly contact each other), thereby further reducing the sourceresistance. Moreover, the leakage current between the source and thegate, and the leakage current between the gate and the drain can bereduced because the GaN cap layer 105 is removed (i.e., the sourceelectrode 106 and the cap layer 105 do not directly contact each other,and the drain electrode 108 and the cap layer 105 also do not directlycontact each other). This is because, as has already been described, thepotential increases suddenly along a direction within the plane asindicated by an arrow a in FIG. 1B due to a potential differenceoccurring in the GaN cap layer 105, so that any electrons contributingto the leakage current will have to acquire a level of energy exceedingsuch increase in potential level. Electrons have an energy of about 26meV at room temperature. When the increase in potential level is 260meV, the leakage current can be decreased by about four orders ofmagnitude, which is an extremely significant reduction. In fact, as canbe seen from the variations in the peak potential in FIG. 3, theprovision of the GaN cap layer 105 with a 10 nm thickness, will allow anincrease in potential level of about 1 eV to be obtained as compared tothe case of not providing the GaN cap layer 105. As a result, a leakagecurrent value is expected to be further reduced.

[0060]FIG. 4 shows a field-effect transistor (FET) 400 which is a firstvariant of Example 1 of the present invention. The field-effecttransistor 400 differs from the field-effect transistor 100 describedwith reference to FIG. 1A in that the field-effect transistor 400 isconstructed so that a portion of a GaN cap layer 405 on which a gateelectrode 407 is laminated is reduced in thickness or removed altogetherby etching. FIG. 4 shows an example of the gate electrode 407 whichcontacts a current donor layer 404. As described above, the GaN caplayer 405 is reduced in thickness or removed altogether and the gateelectrode 407 is laminated in a region where the GaN cap layer hasbecome thin or removed. Therefore, the deterioration of mutualconductance is prevented by the GaN cap layer 405. In this case,although the Schottky barrier height is not enhanced, the increase inpotential level along a direction parallel to the interface between theGaN cap layer and the AlGaN electron donor layer contributes to thereduction of the leakage current.

[0061] In the semiconductor device 100 shown FIG. 1A, an example of thesurface area of the cap layer 105 which is greater than that of the gateelectrode 107 is shown; however, the present invention is not limited tothis. FIG. 5 shows a field-effect transistor (FET) 500 according to asecond variant of Example 1 of the present invention. The field-effecttransistor 500 is different from the field-effect transistor 100described with reference to FIG. 1A in that a GaN cap layer 505 has awidth which is smaller than that of a gate electrode 507. Accordingly,in the field-effect transistor 500, the gate electrode 507 is laminatedin a state extending beyond both sides of the GaN cap layer 505. Effectssuch as reduction in the leakage current and improvement in the voltagebreakdown level can be also attained by using this structure.

EXAMPLE 2

[0062]FIGS. 6A to 6E show cross-sectional views of field-effecttransistors (FET) according to Example 2 of the present invention. Eachof the field-effect transistors shown in FIGS. 6A to 6E includes a GaNcap layer 605 in order to improve the voltage breakdown level.

[0063] A field-effect transistor (FET) 600 shown in FIG. 6A is differentfrom the field-effect transistor (FET) 100 shown in FIG. 1 in that agate electrode 607 provided on the GaN cap layer 605 is disposed closerto a source electrode 606. Therefore, a depletion layer extending acrossa channel layer 603 immediately underlying the gate electrode 607 can belarger on a drain electrode 608 side and the voltage breakdown level ofthe field-effect transistor 600 can be improved.

[0064] A field-effect transistor 610 shown in FIG. 6B is different fromthe field-effect transistor 600 shown in FIG. 6A in that thefield-effect transistor 610 has a structure in which a portion of theGaN cap layer 605 on which the gate electrode 607 is formed is reducedin thickness or removed altogether by etching. In the field-effecttransistor 610 of FIG. 6B, the GaN cap layer is etched so that the gateelectrode 607 contacts a current donor layer 604. In the field-effecttransistor 610 shown in FIG. 6B, the deterioration of mutual conductancecan be prevented by introducing the GaN cap layer 605.

[0065] In a field-effect transistor 620 shown in FIG. 6C, the gateelectrode 607 is provided on the electron donor layer 604 and along aside edge of the GaN cap layer 605 closer to the source electrode 606.Accordingly, the GaN cap layer 605 is disposed between the gateelectrode 607 and the drain electrode 608. In accordance with thestructure of the field-effect transistor 620 shown in FIG. 6C, theleakage current between the gate and the source is not improved, but thevoltage breakdown level between the gate and the drain is improved.Especially, since the gate electrode 607 is formed along the side edgeof the GaN cap layer 605 in a location closer to the source electrode606, the field concentration in the neighboring area of the gateelectrode 607 on the side that is facing the drain can be reduced andthe voltage breakdown level between the gate and the drain can beimproved. In the same manner as in the field-effect transistor 610 shownin FIG. 6B, an increase of the source resistance can be prevented andthe mutual conductance of FET can be improved.

[0066] The above-described example illustrates a case where GaN is usedas the cap layer 605. However, a GaN cap layer 605 cannot be formed witha relatively large thickness. This is because, as shown in FIG. 3, anincreased thickness of GaN makes for insufficient sheet electron densityand/or an excessive peak potential so that positive holes may beaccumulated between the cap layer 605 and the electron donor layer 604.The need for providing a thick cap layer 605 without substantiallyaffecting the sheet electron density is especially pronounced in thefield-effect transistor 620 shown in FIG. 6C. This is because when thecap layer 605 of the field-effect transistor 620 has a large thickness,the field concentration in the neighboring area of the gate electrode607 on the side that is facing the drain is reduced, and the voltagebreakdown level of the field-effect transistor 620 is improved.Moreover, when the cap layer 605 of the field-effect transistor 620 hasa large thickness, a parasitic gate capacitance of the portion where thegate electrode 607 overlaps the cap layer 605 can be reduced, and highfrequency property of the field-effect transistor 620 can be improved.

[0067] There are two methods for providing the thick cap layer 605maintaining properly lowered sheet electron density as follows. A firstmethod is to use an InGaAlN cap layer instead of using the GaN cap layer605. A second method is to reduce a potential difference occurring inthe cap layer by adding an n-type impurity to the cap layer.

[0068] The first method imposes a requirement on the composition ofInGaAlN; that is, the lattice constant of the c facet must besubstantially matched with that of the GaN buffer layer in order toprovide a film having a large thickness. For this purpose, sinceIn_(0.18)Al_(0.72)N and GaN can be lattice matched, a mixed crystal ofIn_(0.18)Al_(0.72)N with GaN may be formed, resulting in(In_(0.18)Al0.72)_(x)Ga_(1−x)N. In practice, however some deviation inthe composition may be allowed. Another requirement is that themagnitude of polarization inside the InGaAlN cap layer must be keptsmaller than that of the polarization occurring in the AlGaN electrondonor layer 604. This imposes a constraint on the value of x in(In_(0.18)Al_(0.82))_(x)Ga_(1−x)N; the maximum value of x depends on theAlN component in the AlGaN electron donor layer 604. The maximum valueof x associated with typical AlN component ratio in the AlGaN electrondonor layer 604 can be calculated as follows: when the AlN compositionof the AlGaN electron donor layer 604 is 10%, the maximum x value isabout 0.16; and when the AlN composition of the AlGaN electron donorlayer 604 is 30%, the maximum x value is about 0.47. The maximum x valuemay be considered to be about 1.5 times the AlN component ratio in theAlGaN electron donor layer 604.

[0069] According to the second method, a proper thickness of the caplayer 605 is determined by the density of the impurity added thereto.Although the material of the cap layer may be GaN or InGaAlN, thefollowing description of the second method conveniently assumes that GaNis used. The thickness of the cap layer can be increased under thefollowing conditions while keeping the potential similar to that in FIG.2 in a region underlying the AlGaN electron donor layer 104 (i.e., aregion spanning a distance of 10 nm or more in FIG. 2).

[0070] In FIG. 2, the surface potential of the cap layer 105 is fixed tothe Schottky barrier height, i.e., 0.76V. An undoped GaN layer can beformed on the cap layer as thickly as possible by performing a doping soas to obtain an electric field which is substantially zero at thesurface of the cap layer 105 and which is equal to the potential (about1.6V) at the interface between the cap layer 105 and the AlGaN electrondonor layer 104. Such requirements are calculated to give 16.7 nm as athickness of the cap layer and 3×10¹⁸/cm³ as a doping density of then-type impurity. An undoped GaN cap layer of a desirable thickness maybe formed on such an n-type GaN cap layer.

[0071] The above-described structure for the cap layer is merelypresented as an exemplary embodiment of the invention, and the actualcap layer can be designed with various combinations of density andthickness. Moreover, as in the field-effect transistors 610 and 620shown in FIGS. 6B and 6C, when the electrical charge control by means ofthe gate electrode mainly occurs at a portion where the gate electrode607 and the field donor layer 604 contact with each other, the cap layer605 may be composed of a combination of a semiconductor layer 605 b(e.g., an n-type GaN layer) with an insulating film 605 a formedthereon, as in the field-effect transistors 630 and 640 shown in FIGS.6D and 6E. A SiO₂ film or a silicon nitride film can be used as theinsulating film. A silicon nitride film is more preferable because it isconsidered to have a relatively low interface level density. Thefield-effect transistor 630 shown in FIG. 6D includes the semiconductorlayer 605 b and the insulating film 605 a provided thereon instead ofthe cap layer 605 of the field-effect transistor 610 shown in FIG. 6B;and the field-effect transistor 640 shown in FIG. 6E includes thesemiconductor 605 b and the insulating film 605 a provided thereoninstead of the cap layer 605 of the field-effect transistor 620 shown inFIG. 6C. In the field-effect transistor 630, the gate electrode 607 isformed so as to contact not only the AlGaN electron donor layer 604, butalso the upper surface of the cap layer 605. It will be appreciatedthat, in the field-effect transistor 610 as well, the gate electrode 607may be formed so as to contact not only the AlGaN electron donor layer604 but also the upper surface of the cap layer 605. Particularly, asdescribed above, the voltage breakdown level can be expected to beimproved by elongating the gate electrode 607 toward the drain electrodeof the cap layer 605.

EXAMPLE 3

[0072] In each of the field-effect transistor (FET) structures describedin Examples 1 and 2, the facet of the heterostructure is composed of aIII group element. However, an alternative structure is required in thecase of forming the facet from a V group element. An example of such aheterostructure, the facet of which is composed of nitrogen, a V groupelement, will be described below.

[0073]FIG. 7 shows a field-effect transistor 700 as a specific exampleof the aforementioned structure. The field-effect transistor 700 isformed on a substrate 701 composed of sapphire or SiC, on which thefollowing layers are sequentially laminated: an AlGaN buffer layer 702having a thickness of about 2 to 3 μm and an AlN component ratio ofabout 0.15 to 0.5; an n-type AlGaN electron donor layer 703, to which ann-type impurity such as Si is added at a density of about 2×10¹⁸cm⁻³; achannel layer 704 formed of GaN or InGaN having a thickness of about 15to 20 nm; and an AlGaN cap layer 705 having a thickness of about 10 nm.In this field-effect transistor 700, the AlN component ratio of therespective AlGaN layers may be the same. However, when polarizationeffects are taken into account, the AlN composition of the surface AlGaNcap layer 705 can be prescribed to be greater than the AlN compositionof the AlGaN buffer layer 702. As in the field-effect transistor 100shown in FIG. 1A, the AlGaN cap layer 705 is selectively removed so asto leave only a central portion thereof. A gate electrode 707 is formedon the AlGaN cap layer 705. The source electrode 706 and the drainelectrode 708 are formed, adjacent to the gate electrode 707, on thechannel layer 704 after the AlGaN cap layer 705 are removed. Asdescribed above, the surfaces of each nitride layer are composed of cfacets of a V group element (nitrogen).

[0074] In the heterostructure field-effect transistor 700 mainlycomposed of GaN, a set of growth conditions by a molecular beam epitaxymethod for forming surfaces of a V group element, has already beenreported. When producing a film so that its surfaces will be formed of aV group element, the direction of polarization occurring in each layeris opposite to that in the case where the surfaces are composed of a IIIgroup element. Whereas the buffer layer 102 of the field-effecttransistor 100 shown in FIG. 1A is composed of GaN, the buffer layer 702of the field-effect transistor 700 is composed of AlGaN. An electrondonor layer 703 including AlGaN to which an n-type impurity such as Siis added and a channel layer 704 are sequentially formed on the bufferlayer 702. Electron supply to the channel layer 704 occurs via the AlGaNelectron donor layer 703 underlying the channel layer 704 as well as viaa positive electrical charge induced by the difference in polarizationbetween the channel layer 704 and the electron donor layer 703.Accordingly, the gate electrode is typically formed directly on thischannel layer 704. The AlGaN buffer layer 702 is formed so as to besufficiently thick so that the lattice strain is reduced. The channellayer 704 including GaN or InGaN is formed so as to be relatively thin,e.g., on the order of tens of nm, since the layer is subjected to acompression strain. As the cap layer 705, AlGaN is used instead of GaN.

[0075] Prevention of an increase of the source resistance and reductionof the leakage current are expected from such structure based on thesame reason described in Example 1.

[0076] A number of variants are possible under Example 2; these variantsare shown in FIGS. 8A to 8E in the form of field-effect transistors(FETs). However, in the field-effect transistors shown in FIGS. 8A to8E, the surfaces of each nitride layer are composed of c facets of a Vgroup element (nitrogen).

[0077] A field-effect transistor 800 shown in FIG. 8A is constructed, asin the field-effect transistor 400 shown in FIG. 4, so that a portion ofan AlGaN cap layer 805 to form a gate electrode 807 is reduced inthickness or removed by etching. Such a structure allows theintroduction of the AlGaN cap layer 805 to prevent the deterioratingmutual conductance.

[0078] A field-effect transistor 810 shown in FIG. 8B corresponds to thefield-effect transistor 500 shown in FIG. 5. In the field-effecttransistor (FET) 810, the gate electrode 807 is formed on the AlGaN caplayer 805, and the AlGaN cap layer 805 has a smaller surface area thanthat of the gate electrode 807. Accordingly, the AlGaN cap layer 805 isformed so as to be within the bounds of the bottom surface of the gateelectrode 807. A reduction of the leakage current and an improvement ofthe voltage breakdown level are expected by forming the field-effecttransistor 810 in the above-described way.

[0079] A field-effect transistor 820 shown in FIG. 8C corresponds to thefield-effect transistor 600 shown in FIG. 6A. The field-effecttransistor 820 has a gate electrode 807 provided on the AlGaN cap layer805, at a different position than in the field-effect transistor (FET)800 shown in FIG. 8A. The region occupied by the ALGaN cap layer 805between the gate electrode and the drain electrode is made larger bydisposing the gate electrode 807 so as to be closer to the sourceelectrode 806. Such a structure allows a depletion layer extendingacross a channel layer 804 directly underlying the gate electrode 807 tobe expanded toward a drain electrode 808 side, and the voltage breakdownlevel of the field-effect transistor 820 can be improved.

[0080] A field-effect transistor 830 shown in FIG. 8D corresponds to thefield-effect transistor 610 shown in FIG. 6B. The field-effecttransistor 830 is different from the field-effect transistor 820 shownin FIG. 8C in that a portion of the AlGaN cap layer 805 where the gateelectrode 807 is formed is reduced in thickness or removed. As in thestructure of the field-effect transistor 830, the deterioration inmutual conductance can be prevented by introducing the AlGaN cap layer805.

[0081] A field-effect transistor 840 shown in FIG. 8E corresponds to thefield-effect transistor 620 shown in FIG. 6C. The field-effecttransistor 840 has a structure in which the AlGaN cap layer 805 isprovided between the gate electrode 807 and the drain electrode 808.Although the structure of the field-effect transistor 840 may have nosignificant effect on the improvement in the leakage current between thegate electrode and the source electrode, it does provide for improvementof the voltage breakdown level between the gate electrode and the drainelectrode.

[0082] Prescribing a large thickness for the cap layer 805 is effectivefor improving the voltage breakdown level between the gate electrode andthe drain electrode in the structure of the field-effect transistor 840.However, when surfaces are composed of a V group element, it is not easyto form a thick cap layer 805 by using any material other than AlGaN.The reason is that, unlike in the case where a facet of theheterostructure is formed of a III group element, due to the compressionstrain experienced by the GaN composing the channel layer 804 inside thefacet, the spontaneous polarization and the polarization caused bypiezoelectric effects occur in opposite directions, and as a whole, thepolarization occurring inside the GaN channel layer 804 has aconsiderably small absolute value. No material that results in a smallvalue of the polarization can be found among materials whose latticeconstants match that of the AlGaN buffer layer 802 except for AlGaN.Accordingly, it is more convenient and easier to dope the cap layer 805as described in Example 2 than to form a thick cap layer by usingmaterials other than AlGaN.

[0083] Moreover, the use of a combination of an AlGaN layer with aninsulating film formed thereon as the cap layer 805, as described inExample 2, is also effective in the structure of the field-effecttransistors 830 and 840. An SiO₂ film or a silicon nitride film can beused as an insulating film, but the use of a silicon nitride film ismore preferable since a nitride silicon film is considered to have a lowinterface level density.

[0084] There have been previous reports on instances in which the GaNbuffer layers 102, 402, 502, or 602, and/or the AlGaN buffer layers 702,802 similar to those described in the present specification are formedon the substrate 101, 401, 501, 601, 701, 801, respectively, with arelatively thin AlN layer (about 100 nm) interposed therebetween. Itwill be appreciated that the present invention is also applicable tosuch instance with no substantial changes.

[0085] The present invention provides a semiconductor device(field-effect transistor), which is capable of: reducing the leakagecurrent as well as preventing an increase of the source resistance ofthe gallium nitride-based heterostructure; and/or improving the voltagebreakdown level as well as preventing an increase of the sourceresistance. As a result, it is possible to improve the powercharacteristics of the semiconductor device of a gallium nitride-basedheterostructure.

[0086] Various other modifications will be apparent to and can bereadily made by those skilled in the art without departing from thescope and spirit of this invention. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the description asset forth herein, but rather that the claims be broadly construed.

1. A semiconductor device comprising: a substrate; a buffer layercomprising GaN formed on the substrate, wherein surfaces of the bufferlayer are c facets of Ga atoms; a channel layer comprising GaN or InGaNformed on the buffer layer, wherein surfaces of the channel layer are cfacets of Ga or In atoms; an electron donor layer comprising AlGaNformed on the channel layer, wherein surfaces of the electron donorlayer are c facets of Al or Ga atoms; a source electrode and a drainelectrode formed on the electron donor layer; a cap layer comprising GaNor InGaAlN formed between the source electrode and the drain electrode,wherein surfaces of the cap layer are c facets of Ga or In atoms and atleast a portion of the cap layer is in contact with the electron donorlayer; and a gate electrode formed at least a portion of which is incontact with the cap layer, wherein the gate electrode is formed on thecap layer, and wherein the gate electrode has a surface area which islarger than that of the cap layer.